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If you have Macintosh filesystem image of an 800k boot disk, but your physical Mac has a 1.44M SuperDrive, how can you produce a physical disk that can boot that Mac?

  • Well, your question is at the top of the search results for "800k floppy vs 1.44m floppy", which will be useful when it's answered. – wizzwizz4 May 21 '16 at 8:28
  • Does copying it to a CD help? – wizzwizz4 May 21 '16 at 8:33
  • This seems really naïve, but have you tried burning it like you would if it were an 800k floppy? AFAIK the Apple Partition Map will just ignore the extra space. – wizzwizz4 May 21 '16 at 10:16
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    A floppy wouldn't have a partition map. The first person who asked the question on a facebook group (which prompted me to begin searching for an answer) suggested that just dd'ing an 800k image to a floppy didn't work, which set me to finding an answer. – Joseph Carter May 24 '16 at 0:06
  • You'd somehow have to copy the 800k image's logical cylinders / heads / sectors (CHS: 80 / 2 / Variable, 8-12) to the 1.44 MB image (CHS: 80 / 2 / 18) to maintain the partition map. According to this Apple doc the tracks/sector counts were: 00-15→12, 16-31→11, 32-47→10, 48-63→9, 64-79→8 – scruss May 24 '16 at 20:12
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Here's what I've been able to determine: A short filesystem on a floppy should work fine, as long as the blocks are in the right order. If the image you're using is a raw image (819200 bytes exactly, often used by emulators, that's easy. Just fdformat the floppy device for 1.44MiB as you would for any other DOS/Windows/Linux machine and write the image using a command like,

dd bs=512 if=floppy_image.img of=/dev/floppy

However, if the image you are using comes from Apple, it will not be raw. It will be either a Disk Copy image (819284 bytes) or a .sea self-extracting disk image, possibly with a BinHex (.bin) wrapper. We'll address that first since it's far more common.

If the image is wrapped in BinHex, we can use The Unarchiver's unar/lsar to unpack the file. Note, you'd be wise to use lsar on the file first as Macintosh files contain characters like spaces rather commonly and unar does not provide an option to rename the output file during extraction.

At this point I recommend renaming or copying the .sea file to one ending in the .dmg extension and running truncate -s 819284 your_image.dmg to produce a Disk Copy image.

Now to convert that to a raw image, you can use dd to trim off the Disk Copy header:

dd bs=84 skip=1 if=disk_image.dmg of=disk_image.img

This leaves you with a raw disk image suitable for use with emulators or writing to a floppy as above. Note, any 800k image written to a floppy this way wastes space on the floppy disk, and the disk requires a 1.44M "SuperDrive" (a name Apple has reused time and again, but in this context refers to machines able to support the 1.44M floppy and equipped with such a drive.) Similar tricks can be used for Apple // ProDOS and GS/OS disk images if the machine is equipped with a SuperDrive and controller or other PC-compatible 1.44MB drive.

There may be other common disk formats I am not aware of for the early Macintosh. Hopefully they can be added to this answer if someone comes across one.

  • so, despite what you said earlier, copying short images to an HD flopy does work after all? – scruss May 25 '16 at 13:12
  • I'm confused. You talk of "The person who asked the original question"; according to this site, that was you. Are you asking on someone else's behalf? Did you copy the text of the question from another site? – scruss May 26 '16 at 8:56
  • I have modified both the question to clarify what was being asked, and the answer to clarify why there are essentially three different kinds of images being talked about here and how to get the one we want out of the ones we're likely to find. Still somewhat new to the site. – Joseph Carter May 26 '16 at 12:07
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To create boot disks for a apple 512k macintosh I had to take a 1.44MB floppy, tape the hole to make it into a 720KB floppy, and then format the floppy as an APPLE floppy and not a PC floppy with a superdrive equipped old world g3 mac tower running osx 10.3. Then I could write the image.

A linux install on an oldworld machine with a superdrive may work to write the image IF there is a way to set the drive to 800KB mode instead of 720KB mode. For slightly newer macs, it looks like you can actually boot from a floppy with a 1.44MB format just by skipping some blocks. http://www.macfaq.org/software/macos.shtml#Q1.1.5

  • Note that if the disk was ever formatted, this is going to likely cause problems later on. The high density formatting will never go away completely, and cannot be restored to virgin state. That means the data written there can and probably will eventually interfere with something. I don't recommend this practice. Find 720k disks if possible, preferably unformatted or already formatted as Mac 800k. Unformatted is easier to find, but these older lower density floppies are more rare now and more expensive. – Joseph Carter Jun 4 '16 at 11:26
  • I did the same trick to get some 1440k floppies to work as 800k disks with a MacPlus and near 50% of the disks failed. Once used as 1440k the magnetic material apparently sets and trying to use it as an 800k floppy is a very bad idea. The worst part is that the reformatted floppies usually work for a few days and then fail mysteriously and completely leaving you in the lurch. – Michael Shopsin Jun 7 '16 at 15:23
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    The boot disk functioned for several months until I sold the computer. I degaused the disks before use with a bulk tape eraser, so maybe that was the key. – silver Sep 8 '16 at 18:19
  • The "magnetic material" on high density (1.2MB, 1.44MB, 2.88MB) floppies is physically different (higher coercivity... or was it remanence ... that oersted magnet-ish-ness business...) out of the box, and will not reliably "take" data from a write head designed for a lower coercivity SD/DD disk. – rackandboneman Sep 17 '17 at 12:00
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Real Mac/IIgs 800k floppy disks cannot be written from Linux because Apple used GCR to get 800k on the disk instead of the 720k that the PC did. You need a Superdrive to write Mac/IIgs format 800k disks.

However as @Mark pointed out you can write 800k disk images to 1440k disks from a PC and a Mac with a Superdrive will read/boot them since the disk images are bits and not sectors. Mac GUI has 800k and 1440k floppy disk images for System 6 and System 7.

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    Where? The original asker was only able to find 800k images. I tried to find them myself, and was unable to find more than a couple. Either way, the disk image is not in GCR format, it's just a filesystem. An 800k filesystem. The question was how do you get that filesystem intact on to a floppy Linux can write and a Mac can read. If it's just the filesystem without a header, dd does the trick, apparently. – Joseph Carter May 26 '16 at 6:15
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    I'm not sure that MacOS will read a 800k disk written on a 1440k disk. You can download any version of MacOS through 7.6 from Mac GUI. I would recommend System 6.0.8 if the computer has less than 4 MB of RAM and no hard disk, and System 7.1 if the computer has a hard disk and 4 MB of RAM. Some games will not run under System 7 but otherwise its a better system and supports many useful features like PC disks, TCP/IP, TrueType, and Quicktime. – Michael Shopsin May 26 '16 at 14:33
  • @MichaelShopsin, as far as I can tell, Apple has used linear addressing for disks since the beginning. Since a disk image is a pattern of bits, not a pattern of magnetic domains, this means that writing an 800K image to a 1440K disk should produce a disk readable in any 1440K drive, though you'll only be able to use the first 800K of the disk. (This is unlike MS-DOS, which stores the physical disk geometry on the disk.) – Mark Jun 4 '16 at 18:32
  • @Mark you may be right, in all my many years as a Mac user I've never written an 800k disk image to a 1440k disk, but it may well work. I remember some warnings in disk utility if you tried to write to the wrong size disk, but that isn't the same thing as not working. 1440k disk images of System 6 and 7 are better than the 800k disk images because they have fuller system installs on them. – Michael Shopsin Jun 6 '16 at 14:20

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